1. Field of Invention
The invention relates to the production method of a liquid crystal display (LCD) and, in particular, to the manufacturing method of a transflective LCD.
2. Related Art
Opto-electronic technologies have recently made tremendous progress, pushing the rapid development in liquid crystal displays (LCD) in the digital era. The LCD has the advantages of high picture quality, small volume, light weight, low driving voltage and power consumption. Therefore, they are widely used in personal digital assistants (PDA), mobile phones, camcorders, notebook computers, desktop displays, vehicle displays, and projection televisions. They have replaced the conventional cathode ray tubes (CRT) and become the mainstream of the industry.
The LCD is the device that utilizes properties of liquid crystal for display. Since they have more flexibility in size and weight than the traditional CRT, the LCD's are commonly used in various kinds of personal systems (from the screens of mobile phones, PDA's, and digital cameras, to the display panels of televisions and advertisement boards).
In the outdoor and strong light environments, the image of the normal transmissive display has a lower contrast. The reflective display, on the other hand, provides better effect and contrast in such environments because it relies on the external sources for display. The reflective display can reduce the use of the backlit mode that consumes a lot of energy. Therefore, it is very suitable for portable electronics. However, it is more difficult for the reflective display to achieve high contrast and high color quality (particularly full colors) at high resolutions. Moreover, when the environmental light source provides insufficient light, the contrast and brightness of the reflective display are greatly reduced. Therefore, it is of great advantages to provide a transflective display using the transmissive technology that provides a backlit source. In this case, it has the advantages of both the transmissive and reflective modes. This is applicable to active driving technologies for amorphous silicon (a-Si) thin-film transistors (TFT) or low-temperature polysilicon TFT's. Consequently, most of the low-voltage information products use this kind of transflective display panels.
The transflective display panel can use the backlit system to compensate for the insufficient environmental light. When the environmental light is sufficient, the transflective display panel does not need to use the built-in light source. Instead, it makes full use of the environmental light and saves the energy by tuning off the backlit system. However, when the cell gaps of the transmissive region and the reflective region are the same, the transmittance vs. voltage curve of the transmissive region is not consistent with the reflectance vs. voltage curve of the reflective region.
If the conventional transflective display uses the single cell gap, then the transmissive region and the reflective region use different control circuits or they use different transistors for control. This may increase the complexity and difficulty in the array and the driving method.
U.S. Pat. No. 6,812,978 discloses a transflective display technology that primarily uses a dual cell gap for the liquid crystal cells or provides a transflective film. FIG. 1 is a schematic view of the conventional display unit with dual cell gaps. As shown in the drawing, a transflective display panel 1 includes an upper substrate 10 and a lower substrate disposed in parallel and a liquid crystal layer 30 inserted into the gap in between. The inner surface of the upper substrate 10, i.e., the one facing the lower substrate 20, contains a black matrix film 12 and a shared electrode 14. The black matrix film is embedded with a color filter layer (not shown). The surface of the shared electrode 14 is provided with an upper alignment film 16. The inner surface of the lower substrate 20, i.e., the one facing the upper substrate 10, has a matrix-form pixel region formed by perpendicularly intersecting electrode lines and data lines.
Each pixel region is controlled by a TFT (not shown) and divided into at least a transmissive region 40 and a reflective region 50. The pixel region includes a transparent electrode 22 on the lower substrate 20 and a passivation layer 24 on the transparent electrode 22. A reflective electrode 26 is provided on the passivation layer 24 of the reflective region 50. A lower alignment film 28 is disposed on the passivation layer 24 and the reflective electrode 26. The liquid crystal layer 30 is disposed between the upper alignment film 16 and the lower alignment film 28.
It is seen in FIG. 1 that within one pixel region, the liquid crystal layer 30 has two regions of different thickness. The cell gap d1 is formed above the reflective electrode 26, i.e., in the reflective region 50. The cell gap d2 is formed above the transparent electrode 22, i.e., in the transmissive region 40. Moreover, d2 is about twice as much as d1. Therefore, the an incident beam penetrates through the liquid crystal layer 30 and reflected by the reflective electrode 26 of the reflective region 50, the optical path is the same as that of the backlit passing through the transmissive region 40. Therefore, the transmittance vs. voltage curve of the transmissive region 40 becomes the same as the reflectance vs. voltage curve of the reflective region 50.
However, designing dual cell gaps for the liquid crystal cells will encounter the problems of a complicated manufacturing process and difficulty in controls. One cannot obtain ideal display effects by simply attaching a transflective film on the display. In view of these problems, it is thus an important subject of the field to provide an optical design needed by the transmissive and reflective regions under the premise of the liquid crystal cell with a single cell gap.